1. Field of the Invention
This invention relates to power transmission belts and, more particularly, to a power transmission belt having a frictional drive surface. The invention is also directed to a rubber composition for use in such belts.
2. Background Art
The ongoing demand for energy conservation has led to the design of highly compact engine compartments on automobiles. High performance engines in these compartments tend to produce a significant amount of heat, thereby requiring the power transmission belts on these engines to operate in a high temperature environment.
Power transmission belts have conventionally been made with the primary rubber component being natural rubber, styrene-butadiene rubber, and chloroprene. However, these rubbers tend to crack prematurely in high temperature environments.
Research has been undertaken for ways to improve the heat resistance of chloroprene rubber. Advances have been made. However, insufficient improvements have been made to make chloroprene rubber functional, to the desired extent, in high temperature environments.
Research has been undertaken using chlorosulfonated polyethylene rubber, hydrogenated nitrile rubber, fluororubber, etc., which have the main chains highly or completely saturated so as to have good heat resistance. Of these rubbers, chlorosulfonated polyethylene is generally recognized as being favorably comparable to chloroprene rubber with respect to dynamic fatigue, wear resistance, and oil resistance. However, chlorosulfonated polyethylene is adversely affected by vulcanization, and particularly by an acid acceptor, with respect to water resistance. With chlorosulfonated polyethylene, acid acceptors are commonly oxides such as MgO, PbO, etc.
Acid acceptors from lead compounds such as PbO, Pb.sub.3 O.sub.4, etc. account for better water resistance in belts. However, lead compounds are not desirable from the standpoint of pollution and hygiene. An acid acceptor of MgO generates MgCl.sub.2 during the cross linking reaction in the belt, as a result of which there is a decline in water resistance, thereby making this acid acceptor generally unacceptable for belt production. On the other hand, a composition with good water resistance is obtainable with the use of an acid acceptor of an epoxy type, rather than the acid acceptors of metallic oxide. However, the epoxy-type acid acceptors cause the emission of a bad odor, which is undesirable.
While power transmission belts made from chlorosulfonated polyethylene may have a good running life and good heat resistance in a high temperature atmosphere compared to belts made from chloroprene rubber, belts with chlorosulfonated polyethylene may have an unacceptably short lifetime in environments in which they are operating at a temperature of less than -30.degree. C.
Presumably, this is a result of the fact that chlorosulfonated polyethylene rubber results from chlorosulfonation of polyethylene, with there being a high cohesive energy of chlorine in the rubber at low temperatures. Thus, the rubber curing takes place at low temperatures, resulting in loss of rubber elasticity. This may result in premature fracture.
Recently, it has been proposed to use an ethylene-.alpha.-olefin elastomer reinforced with a metallic salt of an .alpha.-.beta.-unsaturated organic acid in place of chloroprene rubber to form power transmission belts. Such a composition is disclosed in Japanese Examined Patent Publication No. 9-500930.
In contrast to chlorosulfonated polyethylene, ethylene-.alpha.-olefin elastomers such as ethylene-propylene rubber (EPR), ethylene-propylene-diene rubber (EPDM), etc., are excellent in heat resistance and temperature resistance and are relatively cheap. However, because these compositions do not have a good resistance to oil, they are not commonly used in an environment where oil staining is common. In a dry frictional driving environment, as with a V-ribbed belt, oil staining may make the belts slippery, thereby diminishing their powertransmission capabilities. The use of this type of composition in power transmission belts is disclosed in Japanese Unexamined Patent Publication No. 6-345948.
An ethylene-.alpha.-olefin elastomer, such as ethylene-propylene-diene rubber (EPDM), while excellent in heat resistance and temperature resistance, is generally inferior in wear resistance to chloroprene rubber. With this elastomer, dust worn from the belt may adhere to the belt surfaces. The accumulated dust, which is generated primarily through contact with cooperating pulleys during belt operation, may cause the production of unwanted noise in use.
With an ethylene-.alpha.-olefin elastomer obtainable through sulfur vulcanization, it may be difficult to obtain a sufficient degree of cross linking, which may result in significant wear on the belt in operation. This is particularly so with a V-ribbed belt in which wear dust accumulates between adjacent ribs and tends to promote tack wear of the belt. With an ethylene-.alpha.-olefin elastomer having a large number of double bonds in the molecules so as to increase the degree of cross linking, the problem of tack wear can be reduced to a certain extent, but at the expense of reducing heat resistance.
Ethylene-propylene rubber is generally poor in tear strength, with peroxide vulcanization even further reducing the belt strength so that there is a tendency of load carrying cords therein to separate from the belt during operation. Through sulfur vulcanization, it is difficult to obtain a sufficient degree of vulcanization, with the result being that the belt may wear excessively in use. This is particularly true of V-ribbed belts in which wear dust accumulates between ribs and causes tack wear and potentially significant noise generation. By increasing the degree of vulcanization using EPDM having a large number of double bonds in the molecules, tack wear can be improved, however again with loss of heat resistance.
Typically, with a belt made from chloroprene rubber, a covering canvas has been used that is treated with resorcin-formalin-latex (RFL) and further rubberized to improve moldability and adhesion to a cushion rubber layer. With power transmission effected using the backside of such a belt, the rubber adhesively attached to the covering canvas is prone to being tacky. This may result in excessive frictional wear between the canvas and a cooperating pulley.